1. Field of the Invention
The present invention relates to a display control device having an illuminating function and a storage medium therefor. The control device is applicable to a small-sized portable electronic system such as an electronic notebook or a portable information terminal.
2. Description of Related Art
With development of liquid crystal display devices, semiconductors and electronics techniques, numerous kinds of small-sized multi-functional portable electronic equipment have been commercialized. With such portable electronic equipment, extension of lives of batteries as well as miniaturization is an important factor, and liquid crystal panels, which are low power displays and exhibit excellent display characteristics, are widely used.
However, since the liquid crystal panel is not a self-luminous device, it is difficult to read a screen of the panel under low light conditions. For this reason, a backlight is recently used for illuminating the liquid crystal display panel.
For example, generally adopted is a method of illuminating a liquid crystal display panel using an electroluminescent panel (an EL panel) as a backlight which is mounted on the rear side of the display panel provided with a semi-transmission type reflector.
This method was initially applied to smaller-sized electronic devices such as electronic watches and pagers, and is now used for portable electronic devices having medium-sized screens such as electronic notebooks.
However, the backlight consumes a lot of electric power and, when it is kept on for a long time, a battery life is greatly affected. Therefore, some backlights are designed to emit light of a reduced luminance as low as possible.
In order to reduce power consumption of the backlight, light emission of the backlight is controlled in the following manners:
(1) When an illumination key is actuated, the backlight emits light for a given time and then goes off automatically.
(2) Only while the illumination key is being actuated, the backlight emits light.
(3) Ambient brightness is measured with a brightness sensor. When the brightness falls below a predetermined level, the backlight emits light. Then, when the brightness exceeds the predetermined level, the backlight goes off automatically.
Particularly, with portable electronic devices, the above (1) is widely used since it allows a user to activate the backlight for the given time period only when necessary.
Another method of reducing power consumption of the backlight is disclosed by Japanese Unexamined Patent Publication No. HEI 2(1990)-190895, for example. According to this disclosure, an EL panel is capable of emitting light on a one-character basis, and EL elements are driven only in a region where textual data or drawing data is to be displayed. By use of this method of driving the EL elements, it is possible to save power which would otherwise be necessary for driving EL elements in a region where textual data or drawing data is not displayed.
However, according to the aforesaid method in which the backlight emits light of a reduced luminance, the display is not easy to see in a dark environment even if the backlight is turned on to illuminate the display. Of course, an increase of the luminance will make the display easier to see, but the increase of the luminance will result in a rise in power consumption and therefore a decrease in the battery life.
According to the aforesaid backlight light-emission control (1), the backlight emits light for the given time regardless of the number of displayed characters. Accordingly, for example, in the case of an electronic notebook which has a relatively large screen and may display a lot of characters, the backlight may possibly go off before a user has finished reading all characters. If the backlight light-emission time is set longer, the backlight wastefully consumes the battery by staying on after all characters have been read, in the case where only a small number of characters are displayed.
According to the aforesaid backlight light-emission control (2), the user must keep pushing the illumination key. Therefore, the method cannot be used for an electronic device such as an electronic notebook which requires key operation.
According to the aforesaid backlight light-emission control (3) of automatically turning on the backlight when the ambient brightness falls below the predetermined level by use of the brightness sensor, cost increases. In addition to that, the backlight keeps producing light in dark conditions even when unnecessary, which results in a decrease of the battery life.
According to the aforesaid method disclosed by Japanese Unexamined Patent Publication HEI 2(1990)-190895, a special EL panel is required and the EL panel is driven in a rather complicated manner. Accordingly, cost increases. In addition to that, consideration is not given to effect of an extended light-producing time on the battery life.